heat index," the latest trend inspired by lady gaga. We've ben seeing more of her lately. She and other celebrities are sporting the latest fashion must-have. We have yet to put it here to the "gma" set. The peek-a-boo bra. Abbie boudreau has the latest dos and don'ts. Reporter: Lady gaga, demi lovato, all starting the latest trends. The peek-a-boo bra. It's a great way to give the bra their due. Reporter: Is this trend hot or so not? With the help of our fearless fashionistas, emma and jessica, and long-time fashion guru, gain buckingham, we got schooled in the dos and don'ts. Can you pull off the peek-a-boo bra trend. This is meant to be a little subtle. Reporter: Our first look, not so subtle. I would consider this not only a fashion don't, but a fashion oh, no, what happened to you? Reporter: Oh, no, you didn't. Oh, no, you didn't. This looks like I didn't have the right bra. So, I threw this on for fun. This has to be much more modest up top and on the bottom, as well. Reporter: To avoid a bust, jane says steer clear of push-up dr bras. And like everything in fashion, you have to wear it with confidence. Just like jessica did in this look. I would consider this a do. It's a modest blazer. And the bra is not saying look at me too much. It's not too bright. It is saying look at me. Certainly something that if you run into your dad on the street, you're not too embarrassed to be wearing this. Reporter: My dad would be like, you forgot your shirt. Reporter: A trend you may want to treadlightly. For "good morning america," abbie boudreau. Abc news, los angeles. I don't know. I'm going to recuse myself from this conversation. Smart move. Smart move. I think it -- something to the mystery of fashion. I wonder what's under that camisole? Coming up, alicia keys,

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